Differential accelerometer



May 10, 1 966 P. H. SAVET 3,250,133

DIFFERENTIAL ACCELEROMETER Original Filed June 27. 1960 2 Sheets-Sheet 1 FIGI.

Z??? FIGZ INVENTOR: BY PAUL H. SAVET ATTYS.

United States Patent 3,250,133 DIFFERENTIAL 'ACCELEROMETER Paul H. Savet, Westbury, N.Y., assignor to American Bosch Arma Corporation, Garden City, N.Y., a corporation of New York Original application June 27, 1960, Ser. No. 39,053. Divided and this application Dec. 22, 1964, Ser. No. 420,380

. Claims. (Cl. 73517) This application is a division of my copending application Serial No. 39,053, filed June 27, 1960 and entitled Space Navigation Using Differential Accelerometers.

The present invention relates to accelerometers and has particular reference to accelerometers useful for acceleration gradient measurements.

It has been said that the measurement of acceleration aboard a coasting space vehicle is meaningless since no overall acceleration in a coasting vehicle prevails. However, this statement is true only if the center of gravity of the satellite is considered. Since the gravitational field in which a vehicle is moving is non-uniform, this nonuniformity can be used to provide the information upon which an exploration-of-space trajectory is based.

If, for example, an exploratory space vehicle is circling the earth, the differential acceleration existing at separated locations on the vehicle can be used to provide information leading to the determinationof the orientation of the gravity vectorand the distance of the earths center from the satellite or the gravitational constant of the earth at the position in space occupied by the vehicle. Methods and apparatus for providing such a determination are described and claimed in the above-identified copending application. I

It is an object of this invention to provide new and useful apparatus for detection of differential accelerations.

Another object is to provide a new and useful differ ential accelerometer suitable foruse in producing indicationsof differential accelerations on a space vehicle.

Until the present invention, the gravity gradient has been neglected as a useful reference of space navigation. Although the present state of knowledge may restrict its usefulness to the vicinity of the earth, there is no reason why the same device cannot be used in the gravitational field of other planets as more information concerning these environments is obtained.

There are many specific applications of differential accelerometers to space navigation and exploration which will exemplify the varied uses to which they can be put. For example, differential accelerometers mounted on an inertial platform in a satellite vehicle may be used to slave the platform into continuous alignment with the equipotential surfaces of gravity to thereby continuously titude, position and velocity errors which accumulate during long flight times. In any navigation system, the gradient detector can be used to generate redundant navigational information and thus improve overall accuracy of the system.

For a more complete understanding of the invention reference maybe had to the accompanying drawings, in

which:

FIG. 1 is a diagram, partly in section and partly in block form, illustrating a preferred embodiment of a differential accelerometer in accordance with the invention, connected in a system for its use;

FIG. 2 is an explanatory diagram illustrating the basis for the use of the differential accelerometer on space vehicles;

FIG. 3 is a diagram indicating one instrumentational embodiment including the differential accelerometer;

FIG. 4 is a modification of FIG. 3; and

FIG. 5 is illustrative of one of the computer constructions which may be used in FIG. 3.

Referring first to FIGURE 1 of the diagrams, the basic differential accelerometer is shown. It may not seem to differ in appearance from the earlier vibrating string accelerometer disclosed in copending patent application Serial No. 586,615, filed May 22, 1956 but it does'differ in the physical construction and the external auxiliaryapparatus. In this accelerometer, three similar axially aligned'electrically conducting strings 10, 11, 12 are stretched'between the ends of a frame 13. Interposed between strings 10 and 11 is a mass 14 and interposed between strings 11 and 12 is a mass 15. The three strings 10, 11 and 12 are located in the transverse magnetic fields of the several magnets 16, 17 and 18, respectively. Electronic oscillators 19, 20, 21 are connected across the ends of the wires 10, 11, 12 respectively, and in accordance with Well known phenomena keep the strings vibrating at their natural frequencies. It is known that the frequency of vibration is dependent upon the tension in the strings as Well as the physical characteristics (such as length, cross section, elastic modulus, etc.) of the strings and can be expressed as where K is a constant and T is the tension in the wire.

Assuming that the masses 14, 15 are equal to M and are subject to axial components of tension as defined be low, and that the strings are all identical and not stressed beyond the region of Hooks Law, then where:

T is the initial tension in the wires;

a is the average of the accelerations of the separate masses 14, 15, i.e. acceleration of rigid frame 13, assumed free of extension;

Aa is the differential acceleration between the actual acceleration and the average acceleration of each mass 14,

T T T are the total tensions in wires 10, 11, 12,

respectively;

AT AT AT are the changes in tension of wires 10,

11 and 12, respectively, produced by acceleration;

The difference in the tension of strings 10 and 12 is and the difference between the sum of the tensions in the' outer spring and twice the inner string tension of 1o+ 12 11==2MAa Thus, the tensions in the strings can be employed to measure both the acceleration of frame 13 and the differential acceleration at the positions of the masses 14 and 15. While Equation 2 above (namely T =T,,%MAa) shows that changes in T may be used as a measure of changes in ha, a system based in this relation operates best only when the end strings 10 and 12 are very closely matched,

and hence it is preferred to employ the relationship of Equation 4.

' The string tensions are related to their natural frequency of vibration according to Equation 1 so that in terms of frequency, Equations 3 and 4 become (f12 f1o o and f +f12 f11 Equations 5 and 6 may be reduced to:

(f12 f1o)(f12+f1o)= o and ter string 11 in respective mixers 24, 25 multiplying sums and differences as indicated by Equation 8, accomplished in computers 26, 27 and adding the products together at 28.

A knowledge of the differential acceleration on the two separated masses 14, will be invaluable in outer space navigation and is particularly adapted for control of orbiting vehicles such as satellites as will be described in what follows.

Referring now to FIGURE 2 of the drawings, the theoretical aspects of the use of acceleration gradient effects will be made clear. Let two masses m and m be positioned as at A in FIGURE 2, i.e. along a radial line from the center of the earth with the center of gravity (C.G.) of the masses at a distance r from the center of the earth, and separated by a distance 2L, it will be seen that the mass m is subject to a greater acceleration of gravity than m since m is closer to the center of the earth, and that the differential acceleration on the two masses is directed so as to separate them in the axial direction. In position B where the line joining the masses is perpendicularly disposed to the radial line r, the masses will tend to move toward each other as they are attracted to the center of the earth along radial lines, and the component of the attractive force along the line joining the masses is directed toward the center of gravity from each of the masses.

Intenmediate of these positions, as at C, the lines joining masses m m is displaced by an angle a from the radial line r and this represents the general condition, In the specific condition where 00:54.7 the differential accelera- "tion is zero.

If the system of masses undergoes an angular velocity about an axis through the center of gravity perpendicular to the line joining the masses, an additional differential acceleration equal to the centrifugal acceleration is created. The total differential acceleration therefore is the sum of the gravitational and centrifugal effects.

Navigational apparatus employing the differential acceleration indications can be constructed according to the concepts which follow. Refer to FIGURE 3. A pair of differential accelerometers 30, 31 are mounted on a platform 32 in a manner such that the sensitive axes of accelerometers 30, 31 are perpendicular to each other, and are in the plane of, or parallel to, the platform 32, FIG- URE 3. The rectangles 30a, 31a represent the auxiliary equipment required to convert the tensions of the wires into usable signals and include the auxiliary apparatus of FIGURE 1. The platform is mounted for universal rotation on the frame 33 of the air vehicle which may be a satellite, for example, orbiting around the earth. It shall be assumed that the inertial attraction is negligible between each of the masses and the distributed mass of the vehicle, that the orbital motion is planar and that the accelerometers 30, 31 are kept coplanar with the orbit by control means not shown.

The differential centrifugal effect, along the axis of each accelerometer, of rotation of that accelerometer about a perpendicular axis is equal to twice where L is one half the distance betwen the masses and (d/dt) is the angular velocity of the accelerometer system in inertial space.

A rate gyro 35 mounted on the platform 32 with the input axis perpendicular to the orbital plane (i.e. perpendicular to the plane of the paper in FIGURE 3) continuously measures the angular velocity d/dz of the platform 32 in the plane of the trajectory, and the computer 36 determines d s 2 LG according to the signal obtained from gyro 35. The differential acceleration of each system can be mathematically expressed as:

The outputs of the accelerometers 30', 31 are modified by output of computer 36 to obtain signals:

Since a -j-a is equal to 1r/2, Equations 11, 12 can be solved simultaneously after substitution of for a Thus, by proper mathematical manipulations, it will be found that 13 sin 204 6 1 '3 COS 2112 and = 3 COS 20(2 Now with the value for a or 0: or both, determined in computer 37 equation 11, or 12, can be solved for the value of the ratio of g/r which is proportional to the magnitude of the gradient of the gravity field. Thus, rewriting Equation 12,

2= r 2L(13 cos 2 (14) For instrumentation of the Equation 14, the 0 and a signals previously composed are applied to the inputs of a computer (33) which operates on the input signals according to Equation 14 to produce the desired output signal proportional to g/r.

Thus, the inclination of the platform with respect to the radial line from the earth can be found and the location of the platform along that line also can be found by the diflerential acceleration measurements. This positional information can be continuously monitored and fed to various instruments for correlating environmental data with position, for example, or for use in navigational apparatus.

In an alternate arrangement shown in FIGURE 4, the differential accelerations Aa and Aa are matched and the difference is used to energize a motor 39 which drives the platform 32 in azimuth until the signals M1 and Aa are equal. When this happens, it will be seen that oi is equal to a and the local'vertical lies on the line bisecting the angle formed by the sensitive axes of the differential accelerometers 30 and 31.

- Since 04 and a are equal, each angle must be equal to 45 whence Equations 9 and both reduce to where Aa represents the output of either accelerometer 30,

.31. The angular velocity of the platform is determined by the rate gyro 35 and is determined in computer 36. The output of computer 36 is combined with Aa from either differential accelerometer, for example differential accelerometer 31, in the computer 40 which performs the operations to determine g/ r according to the, equation:

' sion of g/r ratio by the known g value will give the reciprocal of r.

The examples of FIGURES 3 and 4 are merely two of a vast number of instruments which can be based upon the differential accelerometer. The differential accelerometer of FIGURE 1 represents a preferred embodiment of -a differential accelerometer but in its broader form envisions means for detecting the difference in the linear accelerations existing at two points of a vehicle along an axis parallel to the' line joining the two points. This may be accomplished as in FIGURE 1 where the instrument gives the differential acceleration over the short distance of the center string 11, or by Widely separated but accurately aligned accelerometers and means for determining the difference between their outputs. It should be noted that the differential acceleration. is an extremely small value and extremely precise apparatus should be used. For example, the gravity gradient at the surface of the earth is on the order of 10- g/ft., and at thirty-six thousand miles above the earth it is only 10* g/ft., where g is the acceleration of gravity at the surface of the earth.

The constructions of the various computing components shown as rectangles in the figures are not really necessary to an understanding of the invention since the synthesis of computer circuitry for given inputs and outputs according to given mathematical relationships follows a well defined pattern in the art. However, assuming that the diiference acceleration signals which in FIGURE 1 are composed of difference-frequency signals are converted to signals of constant frequency and proportional Computer 36 simply squares the (dqS/dt) signal from gyro 35 and changes it to a level which represents to the proper scaling for the other signals.

As shown in FIGURE 5, computer 37 receives at its inputs signals having magnitudes proportional to d 2 02 [Ada 1 and produces a mechanical signal proportional to 0: according to Equation 13. For example, the ratio of the difference and sum of 0 and 0 is obtained in a dividing circuit 50, the output is divided by three and is matched against the output of aresolver 51. The error signal energizes a motor 52 which drives the resolver 51 through 2:1 gean'ng until the displacement of resolver is 20: and the output of resolver 51 is equal to cos 20: and motor 52 is deenergized.

Computer 38 merely performs the division of 0 by 2L(13 cos a). For example, computer 38 first synthesizes the 2L(l3 cos a signal by using the 20: signal from computer 37 (since and cos ag=+ cos 2:1

and then divides the 0 input by this signal according to Equation 14. Computer 40 simply subtracts the scaled (dqb/dt) and Aa signals according to Equation 16.

In the matter above, it should be noted that in the demonstration of utility, analog computers have been used throughout for ease of description. However, in the practical realization of the circuits, it may be preferable, from an accuracy and weight standpoint, to employ digital computers to perform the required operations.

The possible uses of the differential accelerometers as the primary sensing element are innumerable. It should be noted that thetwo embodiments here described use only one pair of differential accelerometers since the motion is limited to an orbital plane, whereas complete navigational control would require as many as four differential accelerometers. Although the design of navigational instruments of the future may not be specifically taught by this specification, the utility of differential acceleration measurement in such instruments is demonstrated and the scope of the invention should be limited only by the scope of the appended claims.

Equation 2 above may be derived mathematically from fundamental relationships in my differential accelerometer, as follows.

Let:

A10 A A be the respective cross-sectional areas of wires 10, 11 and 12;

L L L be the lengths of wires 10, 11 and 12, respectively;

S S S be theratios of length to cross-sectional areas for wires 10, 11 and 12, respectively;

T be the initial tension in the wires when the accelerations a and a acting on the masses 14 and 15 are zero;

m, m'be the values of mass of the masses 14 and 15,

respectively;

a, a be the accelerations acting on masses 14 and 15,

respectively, along the axis of the wires.

e, e be the displacements of means 14 and 15, respectively, in response to a and a.

In the present of accelerations aand a T10=T0+AT10 where AT AT AT indicate the changes in wire tension due to the accelerations a and a.

Also

From (18) It will be seen that if the movements of m and m are e and e respectively, the extension of the string 10 is e, the extension of string 12 is e' and the extension of string 11 is ee since the frame is rigid and and m-F u-i- 12= The general formulas for total string tensions in identical wires are found by combining Equations 17 and 28. Thus By Hookes law,

E i0- 1o 10 A7111: (ee)E 11 11 21 or L E(6I'-8) =A11AT11=S11AT1) 11 A1112: '6E A12 L12 or E6 AT12=S12AT12=0 where E is Youngs modulus.

From (20) and (21):

m m'iu u+ 12 12= From (22) and (17):

1o( 1o o) 11( 11 o) 12( 12 o) 10 10+ 11 11+ 12 12 1U+ l1+ 12) O Equation 30 is the same as Equation 2 above.

I claim:

1. A differential accelerometer including a pair of weights, a frame, means for supporting said weights in said frame to permit limited linear motion of both of said weights simultaneously with respect to said frame in response to acceleration of said frame, a member attached to and between said weights, means for detecting changes in tension in said member, and means for determining the differential acceleration of said weights from said detected tension changes.

2. A differential accelerometer including a pair of weights, a frame, means for supporting said weights in said frame to permit limited linear motion of both of said weights simultaneously with respect to said frame in response to acceleration of said frame,, a first elastic member attached to and between said weights, a second elastic member connected between said frame and one of said weights, a third elastic member connected between said frame and the other of said weights, said elasticmembers being coaxial with each other, means for detecting changes in tension in said first member, and means for determining the differential acceleration of said Weights from said detected tension changes.

3. A differential accelerometer including a pair of weights, a frame, means for supporting said weights in said frame to permit limited linear motion of said weights, a first elastic member attached between said weights, a second elastic member connected between said frame and. one of said weights, a third elastic member connected between saidfraime and the other of said weights, said elastic members being coaxial with each other, means for maintaining said members vibrating at their natural frequencies, means for deriving signals representative of said natural frequencies of said three members and for combining said signals to obtain a value representative of the differential acceleration of said Weights.

4. A differential accelerometer comprising a pair of Weights, a supporting frame for said weights, means for supporting said weights on said frame to permit linear motion of both of said weights simultaneously with respect to said frame along a line joining said 'Weights in response to acceleration components of said frame along said line, a member attached to and tensioned between said weights so that its tension varies with the differential acceleration of said Weights, and means responsive to changes in tension of said member to produce signals indicative of the differential acceleration of said weights.

5. The accelerometer of claim 4, in which said lastnamed means comprises electrical means for vibrating said member at its natural frequency and for producing electrical signals representative of said natural frequency and hence of said differential acceleration.

References Cited by the Examiner UNITED STATES PATENTS RICHARD C. QUEISSER, Primary Examiner.

5 JAMES J. GILL, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,250,133 May 10, 1966 Paul HO Savet It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, lines 30 and 31 equation (10) for that portion reading "cos 1 read cos 1 same column 4, lines 36 and 37, and lines 40 and 41, equations (11) and (12), for that portion reading "-ZLg", each occurrence, read -2L% same column 4, lines 68 to 70, equation (14), for that portion reading "(l-3 cos 20,2)" read (1-3 cos a column 6, line 28, for "2L(l-3 cos a')" read 2L(1-3 cos 1 line 29, for

"2L(l3 cos 0. read L[1-3 cos a line 37, for

"(d/dt)" read (d/dt) line 56, for "Equation 2" read E ti (2) column 7 lines 41 to 57, equation (21) for t at portion reading "AT =0" read AT column 8, lines 5 and 6, equation (26) for that portion reading "[S read [S ma+ lines 29 to 32, equation [29) for that portion reading "(2m+m')a+" read (2m+m)a Signed and sealed this 1st day of August 1967K (SEAL) Attest:

EDWARD J c BRENNER EDWARD MG FLETCHER,JRm

Commissioner of Patents Attesting Officer 

2. A DIFFERENTIAL ACCELEROMETER INCLUDING A PAIR OF WEIGHTS, A FRAME, MEANS FOR SUPPORTING SAID WEIGHTS IN SAID FRAME TO PERMIT LIMITED LINEAR MOTION OF BOTH OF SAID WEIGHTS SIMULTANEOUSLY WITH RESPECT TO SAID FRAME IN RESPONSE TO ACCELERATION OF SAID FRAME, A FIRST ELASTIC MEMBER ATTACHED TO AND BETWEEN SAID WEIGHTS, A SECOND ELASTIC MEMBER CONNECTED BETWEEN SAID FRAME AND ONE OF SAID WEIGHTS, A THIRD ELASTIC MEMBER CONNECTED BETWEEN SAID FRAME AND THE OTHER OF SAID WEIGHTS, SAID ELASTIC MEMBERS BEING COAXIAL WITH EACH OTHER, MEANS, FOR DETECTING CHANGES IN TENSION IN SAID FIRST MEMBER, AND MEANS FOR DETERMINING THE DIFFERENTIAL ACCELERATION OF SAID WEIGHTS FROM SAID DETECTED TENSION CHANGES. 